In the field of medical technology, various extracorporeal blood treatment apparatuses comprising an extracorporeal blood circuit are known. The known extracorporeal blood treatment apparatuses include for example haemodialysis apparatuses and cell separators, which necessitate an access to the patient's vascular system. In extracorporeal blood treatment, blood is removed from the patient with an arterial puncture cannula via an arterial hose line, the blood being fed back to the patient with a venous puncture cannula via a venous hose line.
In order to convey the blood in the extracorporeal blood circuit, the extracorporeal blood treatment apparatuses generally comprise an occluding hose pump, in particular a roller pump. Occluding hose pumps are also generally provided in the dialysing fluid system of extracorporeal blood treatment apparatuses. From other specialist medical fields, for example in heart bypass operations, other blood pumps are known for the operation of an extracorporeal blood circuit, in particular special centrifugal pumps designed for blood, which are characterised by causing relatively little damage to the blood.
Despite regular monitoring of the vascular access by hospital staff during extracorporeal blood treatment, there is in principle the risk of the venous puncture cannula slipping out of the patient's blood vessel unnoticed. Whereas slipping-out of the arterial cannula is associated with the sucking-in of air into the arterial hose line, the slipping-out of the venous cannula leads to the feared free flow of blood into the surroundings. If the slipping-out of the venous cannula is not detected immediately, therefore, there is the risk of the patient bleeding to death.
Various devices of differing design are known for the monitoring of the vascular access. The known monitoring devices generally rely on the safety devices which are present as standard in blood treatment apparatuses and which, in the event of an incorrect vascular access, trigger an immediate interruption to the blood flow in the extracorporeal blood circuit.
A monitoring device for a vascular access is described in International Patent Publication No. WO 99/29356 A1, wherein the strength of an electric current flowing through the fluid in the hose line is measured. U.S. Patent Publication No. 2004/0254513 describes a monitoring device, wherein the impedance between two electrodes disposed on the arterial and venous hose line is measured. A drawback is that the known devices require the creation of an electrical connection to the fluid flowing in the hose lines.
Monitoring systems are also known for monitoring both the arterial and the venous vascular access, said systems being based on a measurement of the pressure in the extracorporeal blood circuit. A drop in pressure of 20 mmHG can be assumed in practice in the event of the venous puncture cannula slipping out. Since the nominal value of the measurement value resolution lies in practice in the region of 2 mmHG with a maximum total error between 15 and 20 mmHG, the detection of a venous disconnection proves to be difficult. A monitoring system with pressure monitoring is described for example in U.S. Pat. No. 6,221,040. The known pressure monitoring makes use of a special evaluation procedure.
Monitoring devices which can detect the outflow of blood at the puncture point are described in International Patent Publication No. WO 2006/008866 A1 and U.S. Patent Publication No. 2005/0038325. These devices comprise a moisture sensor.